Now, according to reports in the Russian media, the Kremlin is planning to give Syria its own S-300 batteries as well, prompting Israeli Defense Minister Avigdor Lieberman to threaten in an interview on Tuesday: “One thing should be clear – If someone fires on our planes, we will destroy them.”

Lieberman’s message was clear: the Israel Air Force can’t allow itself to lose its freedom to operate in Syrian airspace, even at the price of a diplomatic crisis with Russia.

The IAF’s ability to continue operating over Syria is at the heart of the latest round of hostilities between Israel and Iran. The Islamic Revolutionary Guards Corps is determined to build permanent bases in Syria – as promised to them by the Assad regime in return for supporting the Syrian president through the last seven years of civil war. But the building cannot go ahead as long as Israel is determined to bomb any such bases.

According to a report in the Wall Street Journal last week, the April 9 attack on the T4 air base being used by Iran’s Revolutionary Guards in Syria was carried out by Israel to destroy a Tor air defense system flown in from Tehran. But Russian-supplied batteries would have a different status.

The advanced versions of the S-300 are capable of tracking dozens of airborne targets simultaneously, at hundreds of kilometers. Almaz-Antey, the Russian state-owned manufacturer, claims it can also shoot down cruise and ballistic missiles.

The current air defense systems operated by Syria’s military are of an earlier Soviet generation, and an S-300 would allow it to track IAF aircraft taking off from their bases within Israel.

An S-300 would not mean Israel would lose its strategic advantage. According to foreign reports, the IAF has already closely studied the system’s capabilities and flown “against” it in joint exercises with countries that have bought the system from Russia.

An array of electronic warfare systems have been developed to evade it, and the IAF’s first squadron of F-35I stealth fighters has already been declared operational. The Russian contingent in Syria already operates the S-300 as well as its more advanced version, the S-400.

Transferring an S-300 battery to Syrian control would not necessarily make a major change to the tactical environment in which the IAF already operates. But it would be a powerful strategic statement.

Russia has studiously kept out of the Israel-Iran confrontation in Syria until now. While it has partnered with Iran in keeping Bashar Assad in power, under President Vladimir Putin it is also closely coordinated with Israel.

The Russian balancing act in Syria has meant on the one hand not reining in Iran when it has tried to build a more permanent presence, including on the Syrian side of the Golan Heights; and on the other, not hampering the airstrikes on Hezbollah and Iranian targets that have been ascribed to Israel in the foreign media.

That balancing act is getting more difficult to maintain as hostilities between Israel and Iran become more overt. Supplying S-300s to the Assad regime would be a sign that Putin is getting off the fence and favoring his allies from Tehran over Jerusalem. That in itself would be much more worrying than any new system in the regime’s hands.